In combustion systems such as gas turbines, aircraft engines, rocket motors and heating installations, thermoacoustically induced combustion oscillations can occur as a result of an interaction between the combustion flame and the release of heat associated therewith and acoustic pressure variations. An acoustic excitation can cause the position of the flame, the flame front surface or the composition of the mixture to fluctuate, thereby in turn causing variations in the release of heat. A constructive phase relationship can lead to the occurrence of positive feedback and amplification. Such an amplified combustion oscillation can result in significant noise exposure and damage due to vibrations.
The acoustic properties of the combustion chamber and the boundary conditions present at the combustion chamber inlet and combustion chamber outlet and at the combustion chamber walls have a significant impact on these thermoacoustically induced instabilities. The acoustic properties can be modified by installing Helmholtz resonators.
WO 93/10401 A1 discloses a device for suppressing combustion oscillations in a combustion chamber of a gas turbine installation. A Helmholtz resonator is fluidically connected to a fuel feed line. This causes the acoustic properties of the feed line or of the overall acoustic system to be changed in such a way that combustion oscillations are suppressed. It has nonetheless been shown that this measure is not sufficient in all operating states, since combustion oscillations can still occur even when oscillations in the fuel line are suppressed.
WO 03/074936 A1 discloses a gas turbine having a burner which leads into a combustion chamber at a combustor port, said combustor port being encircled in a ring-like manner by a Helmholtz resonator. By this means combustion oscillations are effectively damped through close contact with the flame, while temperature irregularities are simultaneously avoided. Capillary tubes which effect a frequency adjustment are arranged in the Helmholtz resonator.
EP 0 597 138 A1 describes a gas turbine combustion chamber which has air-flushed Helmholtz resonators in the region of the burners. The resonators are arranged in an alternating manner on the front side of the combustion chamber between the burners. By means of said resonators oscillation energy of combustion oscillations occurring in the combustion chamber is absorbed and the combustion oscillations are attenuated as a result.
By reason of its function each of these resonators has a connecting aperture to the combustion chamber which must be closed by means of a specific air mass. When the resonators are fixed to the combustion chamber wall, this air mass is no longer available for combustion purposes since it is directed past the burner. The flame temperature and the NOx emissions are increased as a result.